The automation of book binding has allowed book production to proceed as an essentially continuous process from the printing and compilation of the pages, or signatures, into book blocks, through the gluing of the block spines, to the application of the cover stock and trimming. Typically, individual pages or signature folds are collated into a compressed stack, or "block". The block may be sewn prior to further processing. Prior to application of the primer adhesive, the outer surface of the spine area can be cut and roughened by a rotating blade to yield an absorptive, planar surface. The block is next provided with a flexible hinge by the application of a coating of a primer adhesive, which may be a latex comprising an emulsified polymeric adhesive resin, or an aqueous animal glue adhesive. The primer is applied in one or more applications by brushing, rolling or a similar process so as to coat and penetrate the roughened spine. A portion of the wet primer wicks between the sheets to form a matrix which when dry, binds the sheets into the body of the flexible film which forms a part of the spine of the block.
The coating of wet primer is next dried and set by exposure of the coating to an intense, high temperature heat source. At least one coating of a covering adhesive, which may be a natural or synthetic animal glue or a polymeric hot-melt type adhesive, is then applied over the dried primer layer to provide the backbone. The cover is adhered to the backbone and the bound pages are trimmed.
The maximum speed at which the adhesive application steps can be accomplished has been limited by the time needed to dry and set the water-based primer layer prior to application of the covering adhesive layer. However, the use of elevated temperatures to speed the drying of the wet primer coatings is limited by a number of factors. Primers based on animal proteins resist moisture release at low temperatures but are subject to extensive thermal degradation at high application or drying temperatures. When dried, these primer layers can exhibit defficiencies with respect to film strength, flexibility and absolute adhesion to the paper.
Latex-based primers skin over when exposed to intense heat sources; the wet coating becomes surfaced with a thin polymeric membrane. The water entrained in the interior of the primer layer vaporizes and lifts this dried membrane into large blisters which can break and burn as they approach the heat source. If the subsequently-applied covering adhesive fails to adhere fully to these blistered or burned layers of primer, the binding process cannot be completed satisfactorily. The damaged, partially-bound books often jam in the assembly line and must be removed and discarded, resulting in substantial economic losses. Furthermore, if the primer coating is not sufficiently dried, application of a hot-melt adhesive at conventional temperatures of about 150.degree.-200.degree. C. can cause splattering and blistering of aqueous primer adhesives.
Recently, improved latex-based aqueous primer adhesives which resist blistering and charring when exposed to high drying temperatures, have been developed which address this problem. See U.S. Pat. No. 4,536,012. These adhesives contain an amount of ungelatinized starch granules effective to complex a major portion of the water in said primer compositions during the drying process. As the temperature of the wet primer coating rises, the starch swells by absorbing contiguous water. At the same time, the primer adhesive coating is coalesced into a uniform plastic film. As the temperature continues to increase, the bound water is released uniformly and relatively slowly from the swollen starch granules. This controlled water release allows the resultant film to resist the formation of large blisters and their subsequent charring. A multiplicity of much smaller bubbles form and burst, cratering the surface of the primer film. The primer coating dries into a uniform, resilient layer which possesses a surface which is well-suited for the application of further adhesive coatings.
In spite of this advance, however, it will be appreciated that the speed of the book binding process would be improved if the need for an intermediate drying step between the application of the primer adhesive and the covering adhesive could be entirely eliminated. Such a simplification would substantially reduce the production time required for each book. It would also allow manufacturers to reduce the energy input required by eliminating the need for thermal energy during the drying step, thereby lowering production costs.
It will be further appreciated that additional advantages can be achieved by reducing the high temperatures required in the application of conventional covering adhesives. For example, in the process disclosed in U.S. Pat. No. 4,536,012, the hot-melt is applied to the dried primer layer at about 150.degree.-200.degree. C. If the covering adhesive could be handled and applied at substantially lower temperatures, the burn hazard to workers would be diminished. In addition, less time would be required to cool the covering adhesive prior to the cover pick up step.